Modern wireless communication systems rely heavily on the generation of accurate frequencies. For example, crystal oscillators are employed to generate reference frequencies for radio transmission and reception as well as to generate timing signals for digital circuits used by the transceiver. Phase locked loops (PLLs), employing a voltage controlled oscillator (VCO) controlled by a reference frequency, are used for frequency synthesis and modulation and other important applications. Although these frequency sources are a practical necessity for current communication systems, they create issues that must be addressed for the communication system to operate correctly. One notable characteristic of oscillating frequency sources such as crystal oscillators and PLLs is the potential for unwanted leakage of energy into other circuit elements, creating spurious signals or “spurs.” The problems posed by these aggressor signals are exacerbated by trends in circuit design to integrate multiple analog and digital functions into a single chip, such as in system-on-a-chip (SOC) applications. As will be appreciated, spurs are particularly problematic when they are manifest in the analog passband and are downmixed with the desired signal, for example. Thus, electromagnetic coupling between the signal path and the frequency source may result in spurious tones appearing at a frequency corresponding to the aggressor signal. Coupling between data sources and the receiver input may also generate multiple spurs or an elevation of the noise floor.
Typically, it is necessary to compensate for these RF spurs to optimize the performance of the wireless system. Conventional techniques for canceling or mitigating these spurs involve a number of tradeoffs. Although the spurs can be processed within the digital baseband, such methods are not entirely satisfactory. For example, filters can be employed in the time or frequency domain to reduce the energy of the signal around the spur frequency. However, this attenuation also impacts the signal being received and is correspondingly detrimental to sensitivity.
Therefore, it would be desirable to provide systems and methods for cancelling or attenuating the spurs in an analog signal path of a wireless communication device. Further, it would be desirable to provide this suppression of the spurs by generating a cancelling signal having characteristics matched to the spur.
This invention accomplishes these and other goals.